Patent Literature 1 discloses the following seat suspension. That is, the seat suspension is configured such that an upper frame provided so as to be vertically movable to a lower frame attached to a vehicle body floor is resiliently supported by a magnetic spring and a torsion bar, and by utilizing such a fact that the magnetic spring has a negative spring constant in a predetermined displacement range and combining the magnetic spring with the torsion bar having a positive spring constant, the whole spring constant is made extremely small, preferably, substantially zero, and it absorbs vibrations.
Further, in such a seat suspension, it is preferred that two magnet sections configuring the magnetic spring and performing an approaching and separating action mutually are held at a neutral position in a state supporting a load (human body) in order to utilize a region where the spring constant of the magnetic spring becomes negative effectively, namely, in order to secure a large vibration absorbing region where the whole spring constant becomes substantially approximately zero as much as possible. Therefore, in Patent Literature 1, a configuration is provided for detecting a vertical displacement of the upper frame when a load mass has been applied to the upper frame, twisting the torsion bar by a predetermined amount, adjusting a pretension of the torsion bar, moving the upper frame vertically, and setting a mutual position between two magnet sections at a neutral position.